1. Field of Invention
The present invention relates generally to the field of command, control, communications, computer, intelligence surveillance, and reconnaissance (C4ISR) hardware and software systems and components, as well as commercial wireless applications and in particular using spread-spectrum communications.
2. Description of Related Art
TTNT (Tactical Targeting Networking Technology) is an advanced tactical data link (TDL) currently under development by Rockwell Collins Government Systems and the Advanced Technology Center. TTNT is an Internet Protocol (IP) based, high-speed, dynamic ad hoc network designed to enable the U.S. military to quickly target moving and time-critical targets. TTNT meets several requirements including transmitting 2 Mbps of data over 100 nautical miles with a latency of less than 2 milliseconds for high priority traffic; 10 Mbps network capacity; Link 16 compatibility; five-second ingress; high Doppler performance and multi-node, beyond-line-of-sight routing. TTNT is intended to support more than 200 users for secure, jam-resistant transmissions at high speed Internet throughputs, and to allow reception of four or more receive streams simultaneously.
In the present invention, certain terms are used, as appreciated by a skilled artisan. Thus “chip” is often defined as “channel bit”. A spread spectrum system, such as used by the present invention, achieves its spectral spreading using one or more techniques such as direct sequence, forward error correction, orthogonal channel coding and frequency hopping. Regardless of the technique used, the bits produced by the spreading are often referred to as “chips”. These chips are modulated and sent over the channel. This distinguishes the bits created by the spreading technique (“chips”) from the information bits going into the spreading technique (“bits”). Note that spread spectrum chips are not required to be binary. “Chip rate” is the rate or frequency at which the chips are transmitted. In a spread spectrum system, the chip rate is much faster than the information bit rate, thus the spectral spreading. “Chip time” is the reciprocal of the chip rate, or the duration in time of a single chip. “Multiple chip times” refers to a period of time that is equal to more than one chip time. A “known sequence” is a sequence of chips (or bits, or symbols) of which an authorized receiver has prior knowledge. The known sequence is typically sent at the beginning of a transmission. The receiver performs a search for the known sequence in order to detect the presence of a desired signal and synchronize its signal processing to it. The process of detecting the presence of a desired signal is often referred to as the signal “acquisition”.
Transmission of a digital signal using continuous bandpass limited signals is also done using any combination of modulation of amplitude, frequency or phase of the sinusoidal carrier wave. The modulating waveform may consist of rectangular pulses, and the modulated parameters, which can be termed symbols, can be switched or keyed from one discrete value to another, using binary or M-ary amplitude-shift keying (ASK), frequency-shift keying (FSK), phase-shift keying (PSK), and the like.
The present invention may be employed in software defined radio (SDR), which employs waveform modulation and demodulation schemes of the kind used in radio data transmission but on a software driven platform, including but not limited to Frequency Modulation (FM), Amplitude Modulation (AM), Single Side Band (SSB), Double Side Band (DSB), Vestigial Sideband (VSB), Frequency Shift Keying (FSK), Phase Shift Keying (PSK), Gaussian Minimum Shift Keying (GMSK), Quadrature Amplitude Modulation (QAM), Frequency Hopped Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), Orthogonal Frequency Division Multiplexing (OFDM) and the like.
Software defined radio (SDR) creates radios that function like computers, where the functionality of a radio is defined by software that can be upgraded, rather than by fixed hardware. SDR has been defined as a radio whose signal processing functionality is defined in software; where the waveforms are generated as sampled digital signals, converted from digital to analog via a high speed Digital-to-Analog Converter (DAC) and then translated to Radio Frequency (RF) for wireless propagation to a receiver. The receiver typically employs an RF subsystem coupled to a high speed Analog to Digital Converter (ADC) that can capture some or all of the channels of the software radio node. The receiver then extracts and demodulates the channel waveform using software executing on a digital processor.
SDR is aimed at solving several of the challenges of over-the-air communications, including compatibility with pre-existing legacy radio systems, ability to emulate transmission and reception of a plurality of different waveforms or forms of modulation (modem control), and more efficient spectrum usage, including operation in different frequency bands, with the lowest possibility of interception, detection and interference from unauthorized parties. One of the first SDRs was the SPEAKeasy SDR, known per se in the art. The GNU Software Radio project (www.gnu.org/software/gnuradio/gnuradio.html) is another well documented SDR initiative.
The US military through the Department of Defense (DoD) has driven the development of next generation SDR with an Open Standard Architecture standard for implementing Joint Tactical Radio Systems (JTRS), which is used to communicate in military communication systems, through the use of an open standard Software Communications Architecture (SCA). The SCA calls out the following features: a Common Open Architecture; the ability to support multiple domains, including airborne, fixed, maritime, vehicular, dismounted and handheld applications; the ability to operate in multiple frequency bands; compatibility with legacy radio systems; the easy ability to upgrade new technologies to improve performance; enhanced security, including cryptographic capability, user identification and authentication, encryption key management, and multiple independent levels of security classification; networking ability, including support for legacy network protocols; software reusability; and support for plug-and-play and real-time reconfigurability, with waveforms being portable from one implementation to another.